Yes, it is at 5-v-5. Is that your hangup? That specialty teams pucks in the net don't count as much and that PP specialist don't get their due and PK specialists should be punished?
Weird argument.
What? No it isn't. +/- counts 5 on 5 goals. It also counts 4 on 4 goals and 3 on 3 goals. This is a problem, though not much of one. It counts empty net goals. This is a large problem. It counts short-handed goals for and against for some reason. This is another large problem. What you end up with is players who play 5 on 5, PK, and when the opposition's net is empty (aka 'defensive defensemen') getting rewarded inordinately because they play in situations where it's hard not to be a +, whereas players who play on the PP but not the PK, and who play when their own net is empty but not when their opponent's is, get punished.
It's remarkable how much of an effect this can have. Bryce Salvador posted the highest +/- of his career in 2011-12 when he was +18. But at 5 on 5 with the goalie in the net, he was -1. He was +19 in all those other situations I talked about. Last season, Damon Severson was -14 at 5 on 5, but he was -17 in all other situations. Now sure, I think Salvador was a good penalty killer and Severson a somewhat lousy power play guy, but I don't think that really accounts for the discrepancy here.
It's a garbage stat if it needs that much interpretation. I'll use +/- when I have to, while having to mention all these caveats. But it's mostly trash, especially over one season.